Firstly, we encouter the problems of crossing the two universes.
Secondly as far as I have seen so far just about no one truely gave this real thought. If by some strange effect the two did meet up and had a scrap, well.
Shivans use subspace drive, which as we all know requires a local stellar body to produce enough gravititation force to allow intra-system jump drives to work. After that specific subspace nodes are required to travel from system to system.
Kushan use hyperdrive, now while this has the advantage of being able to jump just about anywhere does have a limited (if long) range and also requires a slightly longer re-charge time than SS-drives.
Next up is the fact that the Kushan, unlike the GTVA, do have ships comparable in size, power, and 'evilness' to the shivans. I point your attention towards the Avatar heavy cruiser and other super-cap ship.
From looking at the two racial technologies it is probably that the Shivan Sathanas beam cannons are more powerful than ion beams over a period of time. But, the HC is more maneuverable and far faster. This is true for nearly all Kushan ships, they are far far faster than their shivan counterparts. The fastest ship the Shiv's has only travels 90 m/s, the kushan scout does 1000m/s!
And another point, due to the nature of 'no shields' the kushan have, their ships are made far more solidly built and thus have significanly more armour and integrity (a scout can survive an ion beam hit for half a second or so. No shivan vessel (except perhaps the fan made vindhyachel) could survive a main beam cannon hit).
The Shiv vessels also wouldn't appear to have the mass construction capability of a carrier, mothership or commandship. And lets be honest, no meson bomb can compare to the power of the siege cannon.
Over all if both sides where to engage each other in typical combat style of their universe it is anyones guess who would win, but at present the odds are in favour of the Kushan. Although is the situation and location where right either of the two could totally obliterate the other.
That is the fact. And now here is the separate oppinion...
As a hard core HW fan I say... THE KUSHAN WOULD WIPE THE FLOOR WITH THE SHIVANS!!! READ IT AND WEEP!
Thank you for your co-operation.
